Method of producing an anode for a capacitor

ABSTRACT

A method of producing an anode for a capacitor, which includes the steps of molding a continuously deformable material onto a flat anode conductor and simultaneously externally shaping the material, and solidifying the material to form an anode body.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a division of U.S. application Ser. No.09/651,600, filed Aug. 30, 2000.

BACKGROUND OF THE INVENTION

[0002] Field of the Invention

[0003] The present invention relates to a method of producing an anodefor a capacitor.

[0004] In its preferred implementation, the capacitor is a chipcapacitor. However, the present invention can also readily be applied toother capacitors, such as for example, capacitors without housings. Acapacitor without a housing is of low structural height and isintegrated, for example, in a hybrid circuit. However, the followingtext assumes that the capacitor according to the invention is a chipcapacitor.

[0005] Chip capacitors, in particular tantalum chip capacitors, aredistinguished by a high volume-specific capacitance-voltage product,also known as the “CV product.” This means that in these capacitors thevalue of the volume-related product of capacitance and voltage which canbe applied to the capacitor is particularly high. Further advantageousproperties of chip capacitors include a stable thermal behavior andfrequency response, a low residual current and a small loss factor.

[0006] Due to these excellent properties, in particular tantalum chipcapacitors are used for numerous applications in a very wide range offields. New applications, demanding conditions of use and an increasingtendency toward miniaturization in electronics mean that the demandsimposed on chip capacitors are becoming ever greater.

[0007] A pertinent prior art assembly is illustrated in FIGS. 24-26.FIG. 24 shows a diagrammatic section through the structure of aconventional tantalum chip capacitor. FIG. 25 shows a side view of theanode body of that chip capacitor, and FIG. 26 is a plan view of theanode body.

[0008] The prior art chip capacitor comprises an anode body 1, adielectric 2, and a layered cathode 3, which form an actual capacitorelement.

[0009] In addition, there is a housing 4 which is responsible forimportant protective functions for the capacitor element.

[0010] A tantalum wire 5, which in the interior of the housing 4 isconnected to a first metal connector 6. The wire 5 leads to thecapacitor element comprising the anode body 1, the dielectric 2, and thelayered cathode 3. By means of a conductive adhesive 8, the layeredcathode 3 is connected to a second metal connector 7. The second metalconnector 7, similarly to the metal connector 6, leads out of thehousing 4.

[0011] Chip capacitors of this nature are produced in different sizes ofhousing 4, usually with standardized basic surface area dimensions andstructural heights. Consequently, if a higher CV product is to beachieved, the volume taken up by the capacitor element or the anode body1 contained therein must be increased.

[0012] Owing to the use of the tantalum wire 5 in the anode body 1 (inthis respect see, in particular, FIGS. 25 and 26) as the anode-sideconductor, the utilization of the housing can scarcely be increasedfurther. This is because the free end of the tantalum wire 5 is weldedto the metal connector 6 which, in the finished chip capacitor, isintended to provide electrical connection to an electronic circuit on aprinted-circuit board, together with the other metal connector. In adesign of this nature, the distance between the capacitor element andthe housing wall is particularly great especially on the positive side.The distance between the positive metal connector 6 and the capacitorelement or the anode body 1, which is formed by the tantalum wire 5, canscarcely be reduced further, for manufacturing reasons. In other words,the volume of the housing is only insufficiently utilized in the priorart chip capacitor.

[0013] East German Patent DD 215 420 discloses a tantalum chip capacitorin which an anode conductor is embedded in a two-part anode bodyproduced by extrusion. That anode body and anode conductor preassemblyis then sintered. The use of two pre-extruded partial anode bodies hasthe drawback that it is impossible to achieve an exact form fit betweenthe anode body and the anode conductor, owing to manufacturingtolerances. Consequently, the electrical contact between the anode bodyand the anode conductor is impaired.

[0014] German published patent application DE 36 34 103 A1 discloses atantalum capacitor in which a tantalum powder is pressed around a wireanode conductor. That capacitor has the drawback of a small contact areabetween the wire anode conductor and the anode body. The result is anincreased resistance in the capacitor which may have an adverse effecton the electrical characteristics of the capacitor. This is anundesirable effect.

[0015] U.S. Pat. No. 3,903,589 discloses a tantalum capacitor, the anodeof which is produced by immersing the anode conductor in a dispersioncontaining metal powder. When the anode conductor is pulled out of thedispersion, a drop remains hanging from the anode conductor, which isthen dried and sintered. That tantalum capacitor has the drawback thatthe anode body cannot be produced with a defined geometry. Because ofthe absence of an optimized anode geometry and the broad tolerances,that prior art capacitor is subject to poor volume utilization.

SUMMARY OF THE INVENTION

[0016] The object of the invention is to provide a method of producingan anode for a capacitor which overcomes the above-noted deficienciesand disadvantages of the prior art devices and methods of this kind, andwhich has a large contact area between the anode conductor and the anodebody, the anode body of which is of fixedly predetermined shape, and inwhich there is good electrical contact between the anode body and theanode conductor.

[0017] With the above and other objects in view there is provided, inaccordance with the invention, an anode for an electrolytic capacitor,comprising:

[0018] a flat anode conductor; and

[0019] an anode body of a continuously deformable material molded ontothe anode conductor and solidified into a fixedly predetermined shape.

[0020] In other words, the anode has an anode body of fixedlypredetermined shape and a flat anode conductor. The anode body is moldedonto the anode conductor from a continuously deformable material whichcan be solidified.

[0021] There is also provided, in accordance with the invention, acapacitor, comprising:

[0022] the anode according to the above summary, wherein a second endsegment of the anode conductor is shaped into a first terminalconnector;

[0023] a dielectric enclosing the anode body; and

[0024] a layered cathode disposed on the dielectric and connected to asecond terminal connector.

[0025] In other words, the anode body of the capacitor is surrounded bya dielectric, and a layered cathode is provided on the dielectric. Afurther end section of the anode conductor is shaped into a firstterminal connector and the layered cathode is connected to a secondterminal connector.

[0026] The anode according to the invention has the advantage that as aresult of the entire anode body being molded onto the anode conductor inthe form of a continuously deformable material, it is possible to form ahomogeneous anode body which exhibits a good form fit with the anodeconductor and good electrical contact with the anode conductor.

[0027] Furthermore, the anode according to the invention has theadvantage that, as a result of the anode body being shaped with the aidof a continuously deformable material, it is possible to achieve anydesired shape with the aid of suitable molds which are removed before orafter solidification of the anode body. The material from which theanode body is formed may, for example, be a paste containing metalpowder, a green film produced from the paste, or a suitable metal powderitself.

[0028] Due to the flat design of the anode conductor which is sinteredinto the anode body made from sintered tantalum powder, for example, alarger contact area between anode conductor and anode body is achievedcompared with a sintered-in tantalum wire of the same cross-sectionalarea. The number of powder particles which are in contact with thesurface of the anode conductor is increased, and consequently the meanlength of the current paths between the dielectric and the anodeconductor, which comprise tantalum particles which have been sinteredtogether, is reduced. Consequently, it is possible to achieve reducedresistances and an increased capacitance at high frequencies.

[0029] Moreover, the use of a flat anode conductor in the anode bodyreduces the risk of local overheating at the transitions between theanode conductor and a fine network formed by the sintered tantalumparticles when current is flowing. This is because higher currentdensities occur at these transitions than in the adjoining network.Local overheating of this type may be a cause of chip capacitors beingsuddenly and dramatically eroded.

[0030] The primary feature of the capacitor or anode according to theinvention is in particular the production of a strong, large-areaconnection between the anode body, comprising an open-pored sinteredbody which forms the capacitance, and an anode conductor with a largesurface area. For all these components, it is preferable to use tantalumor another suitable metal, such as niobium or a material which allows alayer with a high dielectric constant to be formed.

[0031] Furthermore, an anode in which the anode body completelysurrounds an end section of the anode conductor is advantageous. In thisway, it is possible to achieve optimum utilization or the surface areaof the anode conductor for contact with the anode body. Furthermore,high mechanical stability of the anode is ensured in this way.

[0032] Furthermore, the invention also encompasses a method of producingan anode for a capacitor. The method comprises the following steps:molding a continuously deformable material onto a flat anode conductorand simultaneously externally shaping the material, and solidifying thematerial to form an anode body.

[0033] In a preferred embodiment of the method, the molding stepcomprises applying a paste of a binder system and a powder to the flatanode conductor, and the solidifying step comprises subsequently dryingand sintering the paste.

[0034] In other words, the anode is produced by molding continuouslydeformable material, which can be solidified, onto a flat anodeconductor with simultaneous external shaping, and is then solidified toform an anode body. The simultaneous molding of the material onto theanode conductor and the defining of the external shape of the anode bodywith the aid of a continuously deformable material obviates the need forcomplex post-machining processes for shaping the anode body.

[0035] A process in which a paste which contains a binder system and apowder is applied to the anode conductor and is then dried and sinteredis particularly advantageous. In this process, the paste may be combinedwith the anode conductor to form an anode by means of various methods.By way of example, it is possible to use a paste which is known fromdocument DE 199 27 909 A1 and which comprises a discrete phasecontaining a metal powder and a continuous phase containing organiccompounds.

[0036] The capacitor according to the invention can be used as an SMD(SMD=Surface Mounted Device). The use of a paste simplifies theprocessing of high capacitance and ultra-high capacitance tantalumpowders.

[0037] Furthermore, the invention provides a process for producing theanode according to the invention, in which a powder is pressed around aflat anode conductor, with the anode conductor projecting on one side.Then, the compact is sintered. The method according to the invention mayadvantageously be carried out in such a way that an anode conductor inthe form of a strip-like metal sheet is pushed into a bed of the powderwhich is situated in a press mold, and then the pressing operation takesplace.

[0038] The material tantalum which is preferably used for the anodeconductor is extremely expensive, and consequently the strip used forthe process should be as thin as possible. Conversely, the tantalumsheet used must be sufficiently mechanically stable to be able to bepushed into the powder bed and subsequently bent into a mechanicallystable connector. Within these boundary conditions, strip-like anodeconductors with a width of between 0.3 and 5 mm and a thickness ofbetween 50 and 150 μm have proven suitable. These anode conductors havea width/thickness ratio of between 2 and 100.

[0039] The pressing of the powder may particularly advantageously becarried out by transverse pressing, that is to say by pressingtransversely with respect to the direction in which the strip-like anodeconductor extends.

[0040] The further manufacturing steps correspond to the steps used forthe conventional manufacture of tantalum chip capacitors. Therefore, inthe procedure known as forming, the dielectric is formed from tantalumpentoxide on the inner and outer surfaces of the sintered anode body.After the cathode layers have been applied, cathode terminal and housingare produced. In the configuration according to the invention, the anodeconnector made from tantalum, which is used as the positive electricalterminal, may undergo further treatment in order to be made suitable forsoldering or adhesive bonding, and this statement also applies to theother exemplary embodiments.

[0041] As a modification to the above design according to the inventionof the tantalum chip capacitor, the tantalum powder is also mixed withan additive which, due to its lubricating action, makes the pressingoperation easier and more gentle on the press mold. The flow propertiesof the powder and the mechanical stability of the compact are alsoimproved by the binding action of the additive. Camphor is a standardadditive. Before the compact is sintered, the additive should be removedas far as possible without leaving any residues.

[0042] As an alternative to tantalum, it is also possible to use othersuitable metals, such as for example niobium, or alloys of suitablemetals, or other materials which are capable of forming a dielectric.

[0043] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0044] Although the invention is illustrated and described herein asembodied in a method of producing an anode for a capacitor, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0045] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a diagrammatic vertical section through a firstexemplary embodiment of the invention;

[0047]FIG. 2 is a side view of the anode body of the chip capacitor ofFIG. 1;

[0048]FIG. 3 is a plan view of the anode body.

[0049] FIGS. 4-6 are diagrammatic views corresponding to FIGS. 1-3,respectively, of a second exemplary embodiment of the invention;

[0050] FIGS. 7-13 are diagrammatic illustrations explaining a firstexemplary method of producing an anode body;

[0051]FIG. 14 is a diagrammatic side view illustrating a modification ofthe process of FIGS. 7-13;

[0052]FIG. 15 is a plan view of the structure shown in FIG. 14;

[0053] FIGS. 16-23 are diagrammatic illustrations explaining a secondexemplary process for producing an anode body; and

[0054]FIG. 24 is a diagrammatic section through a conventional tantalumchip capacitor;

[0055]FIG. 25 is a side view of the anode body of the prior art chipcapacitor; and

[0056]FIG. 26 is a plan view of the anode body.

[0057] Identical and functionally corresponding components areidentified with identical reference numerals throughout the drawingfigures. The prior art FIGS. 24-26 are described in the introductorytext above, i.e., under the heading Field of the Invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a chip capacitor assemblyaccording to the invention. The assembly includes a capacitor elementcomprising an anode body 1, a dielectric 2, a layered cathode 3 and aflat anode conductor 9 formed as a flat strip which extends into theanode body 1. The anode body 1 comprises a porous sintered metal bodywhich, as will be explained in more detail below, is constructed from apaste which contains metal powder and is dried and sintered, in theprocess forming a firm, large-area connection to the anode conductor 9.

[0059] The term “flat” in this context connotes a structure with givenarea at which the anode body 1 can adhere. Typically, the flat conductorhas a considerably greater width than height. In the illustratedembodiments, the flat anode conductor 9, at least in its surfaces thatcontact the anode body 1, is wider by a factor of approximately 10-20than its height, or its width is greater than its thickness by at leastone order of magnitude.

[0060] The anode conductor 9 is preferably made from tantalum, which isadvantageously also used as the metal for the metal powder in the paste.

[0061] As in the conventional chip capacitor shown in FIG. 24 thelayered cathode 3 is connected to the metal connector 7 via a conductiveadhesive 8.

[0062] The anode conductor 9 fulfills the functions of the tantalum wire5 and the metal connector 6 in the prior art chip capacitor of FIG. 24:for this purpose, it is bent or shaped into a contact connector (cf. inparticular FIGS. 2 and 3), with the result that the distance between thecapacitor element and the edge of the housing 4 on the positive side canbe reduced. Consequently, the volume of the housing 4 of the componentis better utilized, in order advantageously to achieve a higher CVproduct. Thus, it is possible to select a smaller size of housing 4,while the volume of the anode body 1 remains constant. If the size ofhousing 4 remains the same, the invention allows the capacitor elementor the anode body 1 to be larger than in the prior art (FIGS. 24-26).

[0063] The chip capacitor according to the invention having the flatanode conductor 9 in the anode body 1 makes it possible to produceextremely shallow capacitor elements, as can be seen diagrammatically inFIGS. 4 to 6. FIG. 5, like FIG. 2, shows a side view of the anode body 1with the anode conductor 9, while FIG. 6, like FIG. 3, shows a plan viewof the anode body 1 with the anode conductor 9. However, FIGS. 3 and 6show the entire extent of the anode conductor 9, even though it is infact partially covered by the anode body 1.

[0064] The shallow construction, in accordance with the exemplaryembodiment shown in FIGS. 4-6, results in a particularly large lateralsurface area, which allows short current paths and improves theelectrical properties. If appropriate, the extremely shallow capacitorelements may also be integrated in hybrid circuits without housings.

[0065] To produce an anode body, a paste comprising a binder system anda tantalum powder is printed onto a tantalum foil or a tantalum sheet 10with a thickness of from 50 to 150 μm, using a stencil 11. FIG. 7 showsa plan view of the stencil 11, while FIG. 8 shows a side view of thetantalum sheet 10, together with the stencil 11. In FIG. 9, the stencil11 has been filled with tantalum paste 12. The tantalum sheet 10together with the tantalum paste 12 which has been doctored or printedon, is dried and sintered following the removal of the stencil 11. Thisresults in the assembly shown in side view in FIG. 10.

[0066] Finally, the assembly shown in FIG. 10 is cut to length along thedotted lines (cf. FIG. 11), so that the result is anode bodies which areconnected to the tantalum sheet 10. FIG. 12 shows a side view of ananode body of this type with a tantalum sheet 10, while FIG. 13 shows aplan view thereof.

[0067] If appropriate, the cutting to length to form the individualanode bodies (cf. FIG. 11), given sufficient drying, may also take placeprior to sintering.

[0068] The subsequent process steps correspond to those used for theconventional manufacture of tantalum chip capacitors. Thus, in aprocedure known as forming, the dielectric 2 is formed from tantalumpentoxide on the inner and outer surfaces of the sintered anode body 1.After the layered cathode 3 has been applied, the cathode terminal ormetal connector 7 and housing 4 are produced. The anode connector madefrom the anode conductor 9 made from tantalum, which serves as thepositive electrical terminal, may be treated further for the purpose ofmaking it more suitable for soldering or adhesive bonding.

[0069] As a modification to the exemplary embodiment shown in FIGS. 7 to13, it is possible for the tantalum paste 12 to be printed onto thetantalum sheet 10 on both sides, which may take place in a singleoperation. For printing in two steps, after the first side of thetantalum sheet 10 has been printed, the paste which has been applied tothis side undergoes preliminary drying. Irrespective of whether theprinting is carried out in one operation or in two operations, theultimate result is an assembly as shown in FIGS. 14 and 15. FIG. 14shows a side view and FIG. 15 shows a plan view of the tantalum sheet 10with the anode body comprising two parts.

[0070] The anode body for the chip capacitor can also be produced byscreen-printing, for which purpose a paste comprising a binder systemand tantalum powder is screen-printed onto a tantalum foil or a tantalumsheet 10 with a thickness of from 50 to 150 μm. As in the previousexamples, the tantalum sheet 10 with the screen-printed tantalum paste12 is dried and sintered.

[0071] Following sintering, the tantalum sheet 10 is cut to length. Inthis way, individual anode bodies with an anode conductor comprisingtantalum sheet 10 or tantalum foil corresponding to those shown in FIGS.12 and 13 are obtained. In this case too, the cutting to length, givensufficient drying, may also take place prior to the actual sintering.

[0072] The further process steps are carried out in the manner explainedabove.

[0073] It is also possible to screen-print onto the tantalum sheet 10 onboth sides, if appropriate, in one operation. If the printing is carriedout in two steps, after the paste has been printed on the first side ofthe tantalum sheet 10, preliminary drying may be carried out. In thisway, the ultimate result is the assembly shown in FIGS. 14 and 15,having the tantalum sheet 10 and the tantalum pastes 12 which have beensintered, to form anode bodies.

[0074] In the second exemplary embodiment of the invention which isexplained with reference to FIGS. 16 to 23, an anode conductor iscompletely enclosed by a metal powder paste, so that the anode conductorcan only project out of an anode body 20 produced from the metal powderpaste by drying and sintering on one side, as is shown first of all in aside view and plan view in FIGS. 22 and 23, respectively. To produce anarrangement of this nature, it is possible to employ a multistagestencil process in which initially strips 15 of tantalum foil ortantalum sheet with a thickness of from 50 to 150 μm are placed betweentwo perforated masks 13, 14. FIG. 16 shows a plan view of the perforatedmask 13, together with the strips 15, while FIG. 17 shows a side view ofthe perforated masks 13, 14 together with the strips 15 on a first baseplate 16. That part of the strip 15 which projects into the hole in theperforated masks 13, 14 may be supported by a spacer 17 (cf. FIG. 17).If appropriate, this spacer 17 may also be part of the base plate 16 ormay be fixed thereto.

[0075] After preliminary drying of a paste 18 which has been doctored in(cf. FIG. 18), a further base plate 19 is fitted (cf. FIG. 19), thefirst base plate 16 together with the spacers 17 is removed (cf. FIG.20), and metal powder paste is doctored in a second time (cf. FIG. 21).Following final shaping, drying and sintering are carried out. In thisway, it is possible to obtain the arrangements shown in FIGS. 22 and 23with the anode body 20.

[0076] Alternatively, to produce a chip capacitor, it is also possiblefor a paste comprising a binder system and a tantalum powder to beinjected, pressed or cast around a strip of tantalum foil or tantalumsheet with a thickness of from 50 to 150 μm. Following final shaping,the anode body obtained in this way is dried and sintered. In this way,individual anode bodies with an anode conductor made from tantalum foilor tantalum sheet corresponding to FIG. 22 (side view) and FIG. 23 (planview) are obtained, and these anode bodies are treated further in themanner explained above.

[0077] As a final option for producing an anode body, it is alsopossible for an elastic, film-like mass (green film) to be prefabricatedfrom a paste which comprises a binder system and tantalum powder, andthis mass is then cut to length and adhesively bonded to the tantalumfoil or the tantalum sheet with a thickness of from 50 to 150 μm.Following drying and sintering, individual anode bodies with an anodeconductor made from tantalum foil or tantalum sheet are obtained, asillustrated in FIGS. 22 and 23.

[0078] The anode bodies which are produced by the pressing process usingpowder which is described above look exactly like those illustrated inFIGS. 22 and 23.

We claim:
 1. A method of producing an anode for a capacitor, whichcomprises the steps of: molding a continuously deformable material ontoa flat anode conductor and simultaneously externally shaping thematerial, and solidifying the material to form an anode body.
 2. Themethod according to claim 1, wherein the molding step comprises applyinga paste of a binder system and a powder to the flat anode conductor, andthe solidifying step comprises subsequently drying and sintering thepaste.
 3. The method according to claim 2, wherein the applying stepcomprises printing the paste with a stencil.
 4. The method according toclaim 2, wherein the applying step comprises screen-printing the paste.5. The method according to claim 2, wherein the applying step comprisesplacing the paste on both sides of the anode conductor.
 6. The methodaccording to claim 5, wherein the paste is applied to the anodeconductor via two masks, each assigned to one side of the anodeconductor.
 7. The method according to claim 6, which comprisessupporting an end of the anode conductor, which projects beyond an edgeof a perforation in the mask, with a spacer.
 8. The method according toclaim 2, wherein the applying step comprises a process selected from thegroup consisting of injecting the paste, pressing the paste, and castingthe paste around the anode conductor.
 9. The method according to claim2, which comprises forming a green film from the paste, cutting tolength, and adhesively bonding to the anode conductor.
 10. The methodaccording to claim 1, which comprises pressing a powder around the anodeconductor to form a compact, from which the anode conductor projects onone side, and subsequently sintering the compact.
 11. The methodaccording to claim 10, which comprises pushing the flat anode conductorinto a bed of the powder prior to pressing step.
 12. The methodaccording to claim 10, which comprises providing the anode conductor asa strip-like metal sheet with a width/thickness ratio of between 2 and100.
 13. The method according to claim 10, wherein the anode conductoris selected from the group consisting of a tantalum foil and a tantalumsheet.
 14. The method according to claim 10, wherein the powder is atantalum powder.
 15. The method according to claim 10, which comprisesadding a lubricant to the powder as an additive with a lubricatingaction.
 16. The method according to claim 10, which comprises adding acamphor to the powder as an additive with a lubricating action.
 17. Themethod according to claim 15, which comprises removing the additiveprior to sintering.